Metered output fluid dispensers, and their subset, commonly referred to as pharmaceutical pump dispensers, are well known in the art. Many types of such dispensers such as disclosed in U.S. Pat. Nos. 5,217,148 and 5,279,568 employ a body having first and second interconnected cylinders having different internal diameters with first and second pistons disposed in corresponding cylinders. The first piston is vertically slidable in the first cylinder and remains continuously sealed to the inner wall of this cylinder. The second piston is vertically slidable within the second cylinder and is sealed with the second cylinder throughout the dispensing stroke.
First and second pistons in conjunction with the two body cylinders form a pump chamber whose volume determines the dispenser output. The reciprocal action of the pistons within the cylinders varies the pump chamber volume during alternate compression and suction strokes. Fluid is fed into the chamber via an inlet port and is fed out of the chamber via an outlet port. The inlet port in the aforementioned patent is formed between the second cylinder and second piston when the seal between the second piston and the second cylinder is broken at the end of the suction stroke. The outlet port is configured to open during the compression stroke only when the fluid in the dispenser reaches a predetermined pressure. The part of the compression stroke when dispensing occurs is referred to as a dispensing stroke.
These prior art dispensers employ a plurality of pump components which must be manufactured to very close tolerances in order to establish an accurate volume of the pump chamber and thus the dispenser dose.
The present invention employs new and improved fluid dispenser structures having outputs which are independent of the pump chamber volume and thus reduce the number of components whose tolerances must be critically maintained.